Aluminum alloy for small-bore hollow shape use excellent in extrudability and intergranular corrosion resistance and method of production of same

ABSTRACT

Provided as an aluminum alloy for finely hollow shapes is an aluminum alloy that is reduced in the content of Cu, which is problematic with respect to intergranular corrosion resistance, and that can be kept having a noble self-potential and has excellent extrudability. The alloy has a chemical composition which contains 0.05-0.15 mass % Fe, up to 0.10 mass % Si, 0.03-0.07 mass % Cu, 0.30-0.55 mass % Mn, 0.03-0.06 mass % Cr, and 0.08-0.12 mass % Ti and which optionally further contains up to 0.08 mass % V so as to satisfy the relationship Ti+V=0.08 to 0.2 mass %. Also provided is a process for producing a finely hollow aluminum alloy shape.

TECHNICAL FIELD

The present invention relates to an aluminum alloy which is excellent in extrudability and intergranular corrosion resistance which is used for the extruded small-bore hollow flat tubes which form parts of an aluminum heat exchanger, for example, the condenser, evaporator, intercooler, etc., and a method of production for bringing out the effects of such alloy components.

BACKGROUND ART

In general, an aluminum heat exchanger, for example, a condenser for car air-conditioner use such as shown in FIG. 1, is comprised of tubes through which a refrigerant flows (flat tubes) (see FIG. 2( a)), corrugated fins which exchange heat with air, a tank part of a header pipe, and inlet and outlet members, has means by which members in contact with each other are brazed, and is brazed by using a noncorrosive flux (see FIG. 2( b)).

An aluminum heat exchanger is required to have a high durability. The members which form it, that is, the extruded flat tubes, are naturally required to have corrosion resistance, strength, brazeability, extrudability, etc.

On the other hand, from the viewpoint of the need for reducing the weight of heat exchangers and economy, an aluminum alloy excellent in extrudability which enables complicated small-bore hollow structures and reduced thickness and lighter weight is being demanded. In particular, regarding corrosion resistance, a current theme is the prevention of early penetration defects due to intergranular corrosion which will result in the refrigerant which is sealed inside the flat tubes from flowing out.

As a broadly used method of sacrificial corrosion, there is the method of coating metal Zn on the surface of a flat tube, heating it, and thereby forming a Zn diffused layer at the surface layer part. The Zn diffused layer which is formed is utilized for sacrificial corrosion. (See FIG. 3.)

However, with the effect of sacrificial corrosion by the inclusion of metal Zn, the problem also arises that conversely the Zn diffused layer parts including the joints with the fins are consumed early and therefore the performance of the heat exchanger is reduced.

Numerous aluminum alloys have been proposed for extruded flat tubes where such performance is demanded, but no solution satisfying the demands has been found.

For example, PLTs 1 and 2 propose aluminum alloys which are improved in extrudability and corrosion resistance. These are based on pure aluminum and deliberately add the element Cu and element Fe to improve the extrudability.

CITATIONS LIST Patent Literature

PLT 1: Japanese Patent Publication No. 60-238438A

PLT 2: Japanese Patent Publication No. 5-222480A

SUMMARY OF INVENTION Technical Problem

The alloys which were proposed in PLTs 1 and 2 are indeed improved in extrudability, but are insufficient from the viewpoint of the corrosion resistance. In the above way, in general, the method of sacrificial corrosion has been employed as the means for protecting against corrosion of the flat tubes of a heat exchanger. The aluminum alloys which were proposed in the above PLTs 1 and 2 can be said to be alloys which are considerably noble in terms of potential with a natural potential of about −0.7 VvsSCE, so there is no problem with use at a portion at which the sacrificial corrosion method is employed.

However, the alloys are comprised of pure Al with the element Cu added. Al—Cu intermetallic compounds are formed along the crystal grain boundaries, so corrosion of the crystal grain boundaries is liable to be promoted. That is, at portions of the heat exchanger where measures to reduce the Zn diffused layer or where the sacrificial corrosion method is not used, corrosion of the flat tube itself is liable to proceed.

The present invention was made to solve this problem and has as its object the provision of an aluminum alloy for small-bore hollow shape use which can keep down the content of Cu which poses a problem in intergranular corrosion resistance and maintain the natural potential noble and which is excellent in extrudability.

Solution to Problem

The aluminum alloy for small-bore hollow shape use which is excellent in extrudability and intergranular corrosion resistance of the present invention, to achieve this object, is characterized by having a chemical composition which contains Fe: 0.05 to 0.15 mass %, Si: 0.10 mass % or less, Cu: 0.03 to 0.07 mass %, Mn: 0.30 to 0.55 mass %, Cr: 0.03 to 0.06 mass %, and Ti: 0.08 to 0.12 mass % and has a balance of Al and unavoidable impurities.

Furthermore, it may contain V: 0.08% mass % or less in a relationship giving Ti+V: 0.08 to 0.2 mass.

Note that, the aluminum alloy for small-bore hollow shape use is obtained by homogenization comprising heating a DC cast billet of an aluminum alloy which has the above chemical composition at a speed of 80° C./hour or less to 550 to 590° C., holding it there for 0.5 to 6 hours then holding it at 450 to 350° C. in range for 0.5 to 1 hour or cooling by a cooling speed of 50° C./hour down to 200° C. or less.

Further, a small-bore hollow shape made of an aluminum alloy which is excellent in intergranular corrosion resistance is obtained by reheating the above homogenized billet to 450 to 550° C., then extruding it to a desired shape by a working degree of an extrusion ratio of 30 to 1000.

Advantageous Effects of Invention

The aluminum alloy for small-bore hollow shape use which is excellent in extrudability and intergranular corrosion resistance of the present invention is basically comprised of pure aluminum with contents of Fe, Cu, Mn, and Cr kept low, so the extrudability is excellent. While kept low, the required amounts of Fe, Cu, Mn, Cr, etc. are contained, so the alloy has the strength and corrosion resistance for a small-bore hollow shape for forming a heat exchanger.

In particular, in the alloy of the present invention, the content of Cu is kept to 0.07 mass % or less, so the formation of Al—Cu-based intermetallic compounds is inhibited and the likelihood of intergranular corrosion becomes extremely small. Further, by including a suitable amount of Ti, the dispersion of the element Ti at the grain boundaries or matrix stops the progression of intergranular corrosion and improves the corrosion resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 explains a schematic structure of a general condenser for car air-conditioner use.

FIG. 2 explains a schematic structure of a flat tube and a heat exchanger obtained by assembling the same.

FIG. 3 explains a sacrificial corrosion action of a Zn diffused layer.

FIG. 4 shows a cross-sectional shape of a hollow flat tube for heat exchanger use which is fabricated in the examples.

FIG. 5 compares pressure-time curves at the time of extrusion of homogenized billets in the examples.

DESCRIPTION OF EMBODIMENTS

As explained above, the aluminum alloy for extrusion use which was proposed in PLT 2 is an alloy which is excellent in extrudability, but where the contained Cu forms Al—Cu-based intermetallic compounds at the crystal grain boundaries to cause susceptibility to intergranular corrosion.

Therefore, the inventors engaged in in-depth studies to discover an aluminum alloy which reduces the content of Cu, has an extrudability and mechanical properties equal to the aluminum alloy for extrusion use which was proposed in PLT 2 etc., and is not liable to cause intergranular corrosion.

As a result, they discovered that if keeping down the Cu content to 0.07 mass % or less and, further, suitably adjusting the contents of Fe, Si, Mn, Cr, etc and adding a suitable amount of Ti, this problem can be solved. The details will be explained below.

The aluminum alloy for small-bore hollow shape use which is excellent in extrudability and intergranular corrosion resistance of the present invention has a chemical composition which contains Fe: 0.05 to 0.15 mass %, Si: 0.10 mass % or less, Cu: 0.03 to 0.07 mass %, Mn: 0.30 to 0.55 mass %, Cr: 0.03 to 0.06 mass %, and Ti: 0.08 to 0.12 mass % and further contains in accordance with need V: 0.08 mass % or less in a relationship giving Ti+V: 0.08 to 0.2 mass % and which has a balance of Al and unavoidable impurities.

First, the actions and reasons for limitation of the components will be explained. Below, the indications “%” all are mass %.

Fe: 0.05 to 0.15%

Fe has the action of raising the strength of the aluminum alloy. This action is manifested by the inclusion of 0.05% or more, but if included in more than 0.15%, Al—Fe compounds are liable to be formed and the intergranular corrosion resistance to be detrimentally affected and the extrudability is liable to be worsened, so the upper limit of Fe was made 0.15%.

Si: 0.10% or Less

Si is an unavoidable impurity which enters from the Al matrix, but to keep down the formation of Al—Fe—Si compounds which have a detrimental effect on the workability as well, the upper limit was made 0.10%.

Cu: 0.03 to 0.07%

Cu is an element which is effective for suppressing deep pitting of the Al base material. An effect is recognized by a content of 0.03% or more. However, if the content becomes too great, Al—Cu compounds are formed at the grain boundaries and corrosion from the grain boundaries is promoted. For this reason, the Cu content was made 0.03 to 0.07%.

Mn: 0.30 to 0.55%

Mn has the actions of improving the corrosion resistance and strength, in particular the high temperature strength. These actions are effectively manifested by inclusion of 0.30% or more. Mn increases the strength at a high temperature, so has the great role of preventing large softening at the time of brazing and enabling the rigidity of the structure to be maintained. On the other hand, since the high temperature strength is high, the working pressure at the time of extrusion becomes large and the extrudability falls. Further, Al—Mn-based intermetallic compounds are formed along the crystal grain boundaries and the intergranular corrosion resistance is liable to be detrimentally affected. Therefore, the Mn content was given an upper limit of 0.55%.

Cr: 0.03 to 0.06%

Cr has the action of inhibiting coarsening of the extruded structure. This action is effectively manifested by inclusion of 0.03% or more. However, if included in a large amount, the extrudability is degraded, so the upper limit was made 0.06%.

Ti: 0.08 to 0.12%

Ti refines the cast structure. The distributed state of the Ti element has the action of suppressing intergranular corrosion of the extruded material. This action is manifested effectively by inclusion of 0.08% or more. However, if the content becomes too great, coarse intermetallic compounds are formed and the extrudability is degraded, so the upper limit was made 0.12%.

V: 0.08% or Less

The V and V compounds which crystallize at the time of casting are dispersed in a layer form by extrusion and have the effect of preventing the progression of intergranular corrosion, so this is included in accordance with need. However, if the content becomes too great, the extrudability is degraded, so the upper limit was made 0.08%.

Ti+V: 0.08 to 0.2%

The composite addition of Ti and V enables the effect of suppression of intergranular corrosion to be enhanced, but if the contents of these are too great, the extrudability is degraded, so the upper limit of the total of these was made 0.2%.

The balance consist of unavoidable impurities.

The aluminum alloy for small-bore hollow shape use which is excellent in extrudability and intergranular corrosion resistance according to the present invention is melted by usual means and provided as a billet of a desired shape by the general casting method of the semicontinuous casting method.

In the obtained aluminum alloy billet, to effectively utilize the components contained in the alloy, it is necessary to heat the cast billet at a high temperature to cause the elements forming the intermetallic compounds which crystallized at the time of casting etc. to form a solid solution again in the matrix and otherwise provide uniform distribution of concentration of the added elements as homogenization treatment. As this homogenization treatment, heat treatment at 550 to 590° C. for 0.5 hour to 6 hours is preferable.

The aluminum alloy of the present invention is an alloy comprised of a base of pure Al, restricted in Fe and Si, and having Cu, Mn, Cr, or Ti added.

A billet which is obtained by casting has to be held at 550° C. or more for 0.5 hour or more or else the Al—Fe—Si-based compounds cannot be made to finely disperse and defects arise when extruding the billet by a high working degree. Further, to cause compounds of other elements such as Cu, Cr, and Mn to dissolve in the Al matrix or be present as fine compounds as well, the billet has to be held at 550° C. or more for 0.5 hour or more.

On the other hand, if heating at a temperature over 590° C. for 0.5 hour or more, the Fe, Si, and Cu increasingly dissolve, so this is preferable, but the amounts of dissolution of Mn and Cr become greater, the working pressure at the time of subsequent extrusion ends up becoming increased, and the tendency for the structure of the extruded material to become a coarse recrystallized structure becomes greater.

Further, economically, 6 hours or less is preferable. Long homogenization treatment results in the cost of the billet becoming higher and increased oxidation of the billet surface, so a preferred quality is not obtained. The suitable temperature of the homogenization treatment is 570° C.±10° C. Economically, fast heating and fast cooling are desirable, but in an alloy which contains Mn and Cr, if the temperature is raised to the homogenization treatment temperature by a speed exceeding 80° C./h, large amounts of Mn and Cr will dissolve. A suitable temperature elevation rate is used and sufficient dispersion time is taken from the as-cast state (presence of coarse compounds and large amount of dissolution) until the homogenization temperature so as to make the coarse compounds of Al—Fe, Al—Fe—Si, etc. form solid solutions. On the other hand, the Cr and Mn which had formed solid solutions are made to precipitate as Al—(Fe, Mn, Cr)—Si compounds and Al—Mn compounds to improve the structure of the billet.

On the other hand, by holding the billet at a high temperature, the Mn and Cr compounds dissolve. These have to be made to reliably precipitate as suitable compounds. For this, the billet has to be held at 450 to 350° C. in range for 0.5 to 1 hour or has to be cooled by a gentle cooling rate of 50° C./hour down to 200° C. or less. If off from this condition, Mn and Cr remain in the matrix in solid solutions. Only small amounts precipitate at the time of heating before the later step of extrusion, so the extrusion pressure becomes high and the workability falls.

A material which is obtained by extruding a billet which is obtained by such homogenizing treatment becomes uniform in structures at the surface and inside of the extruded material, and it is possible to suppress coarsening of the crystal grains due to hot working.

Note that, to treat a billet which was obtained by casting the alloy composition of the present invention by predetermined homogenization and obtain the targeted small-bore hollow extruded shape, it is necessary to heat that billet at 450° C. to 550° C. and extrude it by a working degree of an extrusion ratio of 30 to 1000.

If less than 450° C., the extrusion ratio of the small-bore hollow shape is high, so the limit capacity of the extrusion pressure of the extruder (usually 210 kg/cm²) ends up being exceeded and extrusion becomes impossible. Even if able to be extruded, stripping and other defects occur at the inside surface of the small-bore hollow material and further the shape and dimensions become outside the tolerances. Further, with high temperature heating such as over 550° C., the extrusion can be performed easily, but the extrusion ratio and the extrusion speed are high, so during the extrusion, the temperature of the shape material becomes high, a large amount of stripping occurs or local melting occurs at the surface and inside of the small-bore hollow material, and the required shape cannot be maintained. Further, if so small that the extrusion ratio does not reach 30, the characteristic feature of the present invention of the effect of Ti (state where Ti is present inside the shape material in a layer form along the extrusion direction) becomes hard to obtain. Conversely, if trying to make the extrusion ratio over 1000, selection of the die design or extrusion conditions becomes difficult and extrusion itself becomes impossible. Examples

As the fluid for heat exchange use, a Freon-based refrigerant is used. For this reason, as the material which is used for a heat exchanger, an alloy which is excellent in corrosion resistance, strength, and brazeability and further which can be extruded into the main members of parts for assembly of a heat exchanger, that is, 0.5 to 2 mm or so small-bore hollow shapes (flat tubes), is being demanded.

Therefore, the various types of aluminum alloys which have the chemical compositions which are shown in Table 1 were studied for extrusion shapeability, corrosion resistance, strength, and brazeability.

First, the various types of aluminum alloys which have the chemical compositions which are shown in Table 1 were melted and formed into castings of 6 to 10 inch diameters and lengths of 2 to 6 meters.

These castings were homogenized under conditions holding them at 550 to 590° C. for 0.5 to 6 hours, then were heated to 460 to 550° C. and were extruded by extrusion ratio 30 to 1000 dies for forming thin shapes to obtain hollow flat tubes for heat exchanger use which have cross-sectional shapes which are shown in FIG. 4, widths of 16.2 mm, thicknesses of 1.93 mm, and 12 bores of wall thicknesses of 0.35 mm.

Further, the extruded samples were investigated for corrosion resistance, strength, and brazeability. The results are shown in Table 2.

Note that, the strength was judged by the room temperature strength of the annealed material. Based on the 65 MPa of pure Al, examples with a strength over 90 MPa were evaluated as “very good”, examples with a strength of 60 to 90 MPa or so were evaluated as “good”, and examples with a strength of less than 60 MPa were evaluated as “poor”.

The corrosion resistance was judged by evaluating the presence and degree of progression of intergranular corrosion from observation of the microstructure after a corrosion test. Examples where intergranular corrosion was not recognized much at all were evaluated as “very good”, examples with exfoliation corrosion of 100 μm or less were evaluated as “good”, and examples with exfoliation corrosion of 500 μm or more were evaluated as “poor”.

The extrudability was judged by surface defects of the flat tubes (stripping, skin roughness, stripping of relief shapes at inside surface of figure). Examples with no surface defects at all were evaluated as “very good”, examples with slight defects, but not a problem in use were evaluated as “good”, and examples with many surface defects and unable to be used were evaluated as “poor”.

In brazeability, the invention examples and comparative examples were almost equivalent. No differences were found.

Further, as the overall evaluation, passing examples which can be used as flat tubes for heat exchanger use were evaluated as “good” and failing examples which cannot be used were evaluated as “poor”.

TABLE 1 Chemical Composition of Test Materials (mass %) No. Fe Si Cu Mn Cr Ti V 1 0.05 0.08 0.03 0.3 0.03 0.12 Inventive 2 0.05 0.08 0.03 0.07 0.05 0.12 Examples 3 0.13 0.08 0.05 0.55 0.05 0.12 4 0.13 0.1 0.05 0.55 0.03 0.08 5 0.13 0.1 0.05 0.5 0.03 0.08 6 0.13 0.1 0.07 0.5 0.03 0.08 7 0.15 0.1 0.07 0.3 0.05 0.08 8 0.15 0.1 0.07 0.3 0.05 0.08 9 0.15 0.1 0.07 0.3 0.05 0.08 0.08 10 0.15 0.1 0.07 0.3 0.05 0.08 0.05 11 0.2 0.1 0.12 0.2 0.1 0.02 Comparative 12 0.25 0.1 0.15 0.3 0.05 0.02 Examples 13 0.2 0.1 0.07 0.45 0.1 0.02 14 0.2 0.1 0.12 0.1 0.2 0.08 15 0.23 0.1 0.12 0.3 0.03 0.008 16 0.15 0.1 0.07 0.5 0.03 0.008

TABLE 2 Results of Various Evaluations of Test Materials Overall Corrosion Braze- Extrud- evalua- No. Strength resistance ability ability tion 1 Good Very good Good Very good Good Inventive 2 Good Very good Good Very good Good Examples 3 Very good Very good Good Good Good 4 Very good Good Good Very good Good 5 Very good Good Good Good Good 6 Very good Good Good Good Good 7 Good Very good Good Very good Good 8 Good Good Good Very good Good 9 Good Very good Good Very god Good 10 Good Very good Good Very good Good 11 Poor Poor Good Poor Poor Compar- 12 Good Poor Good Poor Poor ative 13 Good Good Good Poor Poor Examples 14 Good Good Good Poor Poor 15 Good Poor Good Good Poor 16 Good Poor Good Good Poor

Next, the homogenization conditions were verified.

Aluminum alloys of the chemical composition shown in No. 2 of Table 1 was melted, degassed, refined, filtered, and otherwise treated, then was cast by the DC casting method at 680° C. or more in temperature to a diameter 210 mm billet.

After that, it was homogenized by holding at 590° C. in temperature for 4 hours, then cooling to thereby obtain a 4000 mm length billet. This billet will be called the “current treatment billet”.

As an invention example, such cast billet was heated to the homogenization temperature of 590° C. in the process of which (temperature elevation process) the temperature elevation rate was made 80° C./hour or less, was further held at the homogenization temperature for 4 hours, then was cooled, during which it was cooled in the range of 450 to 350° C. by a 50° C./hour cooling rate, then was cooled outside the furnace. This billet will be called the “present invention billet”.

If examining the microstructures of the current treatment billet and the present invention billet, the current treatment billet was not observed to have any notable compounds other than crystals of the billet before treatment, while the structure of the present invention billet was observed to have precipitates (Al—Mn based) finely dispersed in addition to crystals.

Next, these two types of billets were made lengths of 500 mm and extruded by an extrusion ratio of 150 to produce flat tubes of the shape such as in FIG. 4 at 500° C. by a 15 m/min extrusion speed. At this time, the extrusion pressure at the time of extrusion was measured.

The results are shown in Table 3. Note that, FIG. 5 shows a comparison of pressure-time curves at the time of extrusion.

The method of the present invention is smaller in highest pressure and faster in pressure drop compared with the current method. Further, it is learned that the area inside of the curve is small and the energy which is required at the time of extrusion is small.

TABLE 3 Current homogenizing Homogenizing treatment of treatment present invention Microstructure of Only crystals Crystals and precipitates billet structure Extrusion 210 kg/cm² 180 kg/cm² pressure Extruder capacity: limit Extrusion reduced 15% pressure from limit pressure

Furthermore, the extrusion ratio at the time of extrusion was verified. The present invention billets which were used for verification of the above homogenization conditions were heated to 480° C., then flat tubes of an extrusion ratio of 150 and flat bars of an extrusion ratio of 20 were extruded by a speed of 20 m/min. These two types of extruded materials were observed for microstructure and the states of dispersion of the Ti compounds were compared. The results are shown in Table 4.

TABLE 4 Extrusion ratio 150 Extrusion ratio 20 Microstructure of Crystal grains are Crystal grains are extruded material microstructures microstructures State of Ti Ti distribution is laminar Ti distribution is thin. dispersion of dispersion along long Laminar dispersion difficult extruded direction of extruded to find. material material

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided, as an aluminum alloy for small-bore hollow shape use, an aluminum alloy which can keep down the content of Cu which poses a problem in intergranular corrosion resistance and maintain the natural potential noble and is excellent in extrudability. 

1. An aluminum alloy for small-bore hollow shape use which is excellent in extrudability and intergranular corrosion resistance which has a chemical composition which contains Fe: 0.05 to 0.15 mass %, Si: 0.10 mass % or less, Cu: 0.03 to 0.07 mass %, Mn: 0.30 to 0.55 mass %, Cr: 0.03 to 0.06 mass %, and Ti: 0.08 to 0.12 mass % and has a balance of Al and unavoidable impurities.
 2. The aluminum alloy for small-bore hollow shape use which is excellent in extrudability and intergranular corrosion resistance as set forth in claim 1, further containing V: 0.08% mass % or less in a relationship giving Ti+V: 0.08 to 0.2 mass %.
 3. A method of production of an aluminum alloy for small-bore hollow shape use which is excellent in extrudability and intergranular corrosion resistance comprising homogenization by heating a DC cast billet of an aluminum alloy which has a chemical composition as set forth in claim 1 at a speed of 80° C./hour or less to 550 to 590° C., holding it there for 0.5 to 6 hours then holding it at 450 to 350° C. in range for 0.5 to 1 hour or cooling by a cooling speed of 50° C./hour down to 200° C. or less.
 4. A method of production of a small-bore hollow shape made of an aluminum alloy which is excellent in intergranular corrosion resistance comprising homogenization by heating a DC cast billet of an aluminum alloy which has a chemical composition as set forth in claim 1 at a speed of 80° C./hour or less to 550 to 590° C., holding it there for 0.5 to 6 hours then holding it at 450 to 350° C. in range for 0.5 to 1 hour or cooling by a cooling speed of 50° C./hour down to 200° C. or less, reheating the billet to 450 to 550° C., then extruding it to a desired shape by a working degree of an extrusion ratio of 30 to
 1000. 